This is a project I started roughly a year ago. It still has some rough edges with documentation, but the core is solid and usable.
The core of the engine is based on the JVM and uses Akka, which opens up interesting possibilities for java games. The server side is fully distributed, and a client in java can basically be a member of the cluster, making a fully distributed architecture end to end.
But anyways the best thing is to just check out the project, it's on github:https://github.com/gamemachine/gamemachine
Follow the getting started guide to get a working server. While the only official client I have right now is in C#, java integration should be straight forward. I'd be happy to offer pointers on how to go about that and get it working with minimal effort.